The present invention relates to a hydraulic pressure control device for use in dual brake circuits of a motor vehicle.
In a brake system of the so-called X-pattern fluid circuit type, dual hydraulic fluid paths are provided. Each of the fluid paths is connected to each one front wheel brake and one rear wheel brake positioned on opposite sides of the vehicle. According to the conventional X-pattern fluid circuit, generally, dual control valves are provided to independently control hydraulic pressure application to the right and left rear wheel brakes in order to obviate precedence lock of the rear wheels with respect to the front wheels. With this structure, a safety braking operation is obtained when the brake system of the vehicle fails. However, this arrangement is costly, and a pressure differential may occur between the two paths. Therefore, recently a single control valve has been provided to control the hydraulic pressures applied to the two paths so that they are approximately equal. According to one example of a dual fluid pressure control valve of this kind, a pressure reducing valve is disposed only in the first brake path and a pressure responsive valve is provided in the second brake path. The pressure responsive valve is arranged in series with the pressure reducing valve. The pressure responsive valve is adapted to output a hydraulic fluid pressure equal to the fluid pressure of the first brake path in response thereto.
According to the control valve thus constructed, if no pressure increase occurs upon a fault in the first brake path, the output fluid pressure in the second brake path should preferably be made higher than that in its normal operation. However, disadvantageously, such an output pressure increase is prevented so that it is impossible to compensate for the shortage of the braking force.
Further, a control valve for which the above disadvantages are eliminated is disclosed in Japanese Laid-Open Utility Model Application No. 54-48281. This control valve is provided with a third piston (intermediate cylinder) at an outer peripheral surface of a follower piston. The third piston moves to the fault side upon the occurrence of fault in the first brake path in order to release a bypass passage connecting input and output fluid pressure chambers of the second brake path. The bypass passage is normally closed, but upon movement of the third piston, it is opened to increase the output fluid pressure.
This control valve, however, has the following drawbacks:
(1) Since the third piston is provided at the outer periphery of the follower piston, the control valve is bulky as a whole.
(2) Upon the occurrence of a fault in the first brake path provided with the pressure reducing valve, the third piston, which has a large diameter, is moved to the fault side. Therefore, the amount of compensation of the second brake path is increased. As a result, a shortage in the stroke length of a fluid pressure generating device such as the master cylinder occurs.